With increasing spectral crowding at lower frequencies, microwave communications have become a variable alternative and present some interesting opportunities. However, microwave communications have their own set of particularized problems that need to be resolved before extensive commercialization of microwave communications can be realized.
Microwave filter design is but one of those problems to be resolved.
More particularly, in microwave communications, where the microwave frequency spectrum must be heavily subdivided, microwave filter design has become particularly troublesome.
Microwave waveguide dielectric resonator filters have been employed to perform bandpass and band reject functions. Ordinarily, a waveguide of rectangular cross section is provided with a dielectric resonator that resonates at a single center frequency as it is excited by the microwave electromagnetic field. The center frequency of the filter can be set in various ways. The center frequency can be changed by introducing a disturbance in the electromagnetic field about the dielectric resonator or by altering the mass of the resonator.
The response characteristic of the filter can be altered by introducing a number of dielectric resonators in proximity with each other such that the radiated energy coupled from one resonator to the next alters the bandwidth of the filter. It is well known that the bandwidth of a filter is a function of the product of the resonant frequency of the filter and the interresonator coupling coefficient-a coefficient of the energy coupled between resonators. In dielectric resonator filters, the interresonator coupling coefficient can be changed in a variety of ways.
In an evanescent mode waveguide (a waveguide below cut off), dielectric resonators are usually cascaded at the cross sectional center line in a rectangular waveguide (i.e. at the electromagnetic field maxima). To achieve a certain, desired bandwidth, the resonators are longitudinally spaced to provide the desired interresonator coupling. Since the bandwidth is a function of both interresonator coupling and center frequency, a different spacing between resonators (interresonator spacing) is required for each center frequency to maintain the desired filter bandwidth. Accordingly, the cumulative filter length is different for each and every center frequency. Therefore, heavy subdivision of a frequency spectrum results in a multiplicity of filter lengths, corresponding component parts, and manufacturing fixtures.
To eliminate the multiplicity of filter lengths required to service any frequency spectrum, tuning devices were injected to disrupt the energy coupled between resonators (interresonator coupling), thereby providing a tunable bandwidth. However, tuning could only be performed over a relatively small range of frequencies. Also, in multiple pole filters, tuning became an extremely sensitive and laborious task due to the large number of bidirectional and cumulative interresonator couplings and the interaction with the multiple tuning devices.
One of the inventions, presented herein, solves the tuning problem by fixing the interresonator spacing and altering the interresonator coupling coefficient by simultaneously adjusting the position at which the resonators intercept the electromagnetic field distributed across the waveguide cross section.
Another invention, presented herein, solves the tuning and multiple length problem by determining the combination of interresonator spacing and electromagnetic field interception positioning such that changes in interresonator coupling are inversely proportional to changes in center resonant frequency. This inverse proportionality compensates for frequency changes such that filter bandwidth remains constant over the entire frequency spectrum of interest.
This invention represents a significant advance over the prior art and over this technical field by providing a single filter structure that can be utilized throughout the frequency spectrum of interest without resorting to extensive tuning, but remains substantially set to the proper bandwidth whenever the center frequency is changed.